The Japan Transport Safety Board has concluded its investigation into the overheating of the lithium-ion main ship battery aboard a Boeing 787 last year without reaching a definitive conclusion on the cause. However, the report, issued on Thursday, said that “inappropriate” testing might have contributed to the Jan. 16, 2013 incident, which led to a worldwide grounding of the Dreamliner fleet until Ethiopian Airlines resumed service on April 27 that year. It also pointed to low temperatures as a possible culprit due to lithium metal’s tendency to form deposits on a battery’s electrodes in such conditions.
In its summary of probable causes, the JTSB said internal heat generation in one of the battery's cells likely developed into venting, resulting in cell-to-cell propagation and subsequent failure of the battery. Internal heat generation and internal pressure likely caused the cell to swell, melt the surrounding insulation and contact the brace bar, creating a grounding path that allowed high currents to flow through the battery box, it added.
An internal short circuit likely caused heat to generate in the cell, identified as cell 6, said the report. Although it did not identify the source of the short circuit, the JTSB said that a test conducted during the design’s development did not appropriately simulate the on-board configuration and that designers underestimated the effects of an internal short circuit.
According to the report, an internal short circuit test by nail penetration “very likely” did not produce thermal runaway because the battery box was not grounded with a ground wire. “This demonstrates that it was inappropriate to exclude the internal short circuit test from the safety assessment based on the test result, which was not conducted simulating the actual aircraft configuration,” said the report.
The JTSB issued several recommendations to the U.S. Federal Aviation Administration, including guidance to review the technical standards for lithium ion batteries to ensure appropriate simulation of the electrical environment.
